The present invention relates to the field of measurement and nondestructive testing, and provides an automated method and apparatus for determining the shape of a countersunk hole or a fastener head. More generally, the invention can be used to characterize any surface having variations in depth.
The present invention is especially useful in fields where fastener holes must be formed with precision. For example, the exterior surface of an aircraft is made of a plurality of aluminum or composite panels, the panels being held together by rivets or other fasteners. Countersunk holes are essential for the fasteners to achieve maximum strength. If the orientation of the fastener does not exactly coincide with that of the hole, the result can be an uneven stress concentration, and cracks can form and propagate from one hole to another. Mismatches in orientation between the fastener and the hole can result from holes which are drilled too deeply or not deeply enough, or from holes which are drilled at an angle instead of being normal to the panel surface. The problem of stress concentration is especially critical in the case of composites. However, the problem occurs with metal panels as well.
Countersunk holes are also used to minimize aerodynamic drag, as such holes enable the fastener heads to be positioned at or below the level of the surface of the panel. If the fastener holes are too shallow, the fastener heads will protrude beyond the surface, and will create excessive drag at high airspeeds.
The need for precision is not limited to countersunk holes, but also applies to fastener heads. If the fastener head is not shaped correctly, it may not fit perfectly within the fastener hole, thus compromising the strength of the joint. Also, if the fastener head is too large, it will protrude unduly, again causing unwanted drag.
The above-described problems can occur wherever fasteners must be inserted into holes with great precision. In high performance aircraft, even a slight protrusion of a few rivets can have a perceptible effect on the flying characteristics of the aircraft, and on its structural strength under high loads.
Thus, there is a need to know whether the dimensions of a countersunk hole, or of a fastener head, are within allowable limits. Since a given structure usually has a very large number of fasteners and holes, testing all of them can be difficult and time consuming. More significantly, all known prior art systems for performing such analysis are subject to errors caused by misalignment of the components used to perform the measurements.
The present invention provides a method and apparatus for determining the shape of a countersunk hole, and/or the shape of a fastener inserted in such a hole. The method and apparatus of the invention provide a nondestructive, automatic, and accurate technique for performing such analysis.